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91 lines
3.9 KiB
91 lines
3.9 KiB
RCU and lockdep checking
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All flavors of RCU have lockdep checking available, so that lockdep is
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aware of when each task enters and leaves any flavor of RCU read-side
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critical section. Each flavor of RCU is tracked separately (but note
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that this is not the case in 2.6.32 and earlier). This allows lockdep's
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tracking to include RCU state, which can sometimes help when debugging
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deadlocks and the like.
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In addition, RCU provides the following primitives that check lockdep's
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state:
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rcu_read_lock_held() for normal RCU.
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rcu_read_lock_bh_held() for RCU-bh.
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rcu_read_lock_sched_held() for RCU-sched.
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srcu_read_lock_held() for SRCU.
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These functions are conservative, and will therefore return 1 if they
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aren't certain (for example, if CONFIG_DEBUG_LOCK_ALLOC is not set).
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This prevents things like WARN_ON(!rcu_read_lock_held()) from giving false
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positives when lockdep is disabled.
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In addition, a separate kernel config parameter CONFIG_PROVE_RCU enables
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checking of rcu_dereference() primitives:
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rcu_dereference(p):
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Check for RCU read-side critical section.
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rcu_dereference_bh(p):
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Check for RCU-bh read-side critical section.
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rcu_dereference_sched(p):
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Check for RCU-sched read-side critical section.
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srcu_dereference(p, sp):
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Check for SRCU read-side critical section.
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rcu_dereference_check(p, c):
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Use explicit check expression "c". This is useful in
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code that is invoked by both readers and updaters.
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rcu_dereference_raw(p)
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Don't check. (Use sparingly, if at all.)
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rcu_dereference_protected(p, c):
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Use explicit check expression "c", and omit all barriers
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and compiler constraints. This is useful when the data
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structure cannot change, for example, in code that is
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invoked only by updaters.
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rcu_access_pointer(p):
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Return the value of the pointer and omit all barriers,
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but retain the compiler constraints that prevent duplicating
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or coalescsing. This is useful when when testing the
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value of the pointer itself, for example, against NULL.
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The rcu_dereference_check() check expression can be any boolean
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expression, but would normally include one of the rcu_read_lock_held()
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family of functions and a lockdep expression. However, any boolean
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expression can be used. For a moderately ornate example, consider
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the following:
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file = rcu_dereference_check(fdt->fd[fd],
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rcu_read_lock_held() ||
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lockdep_is_held(&files->file_lock) ||
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atomic_read(&files->count) == 1);
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This expression picks up the pointer "fdt->fd[fd]" in an RCU-safe manner,
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and, if CONFIG_PROVE_RCU is configured, verifies that this expression
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is used in:
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1. An RCU read-side critical section, or
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2. with files->file_lock held, or
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3. on an unshared files_struct.
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In case (1), the pointer is picked up in an RCU-safe manner for vanilla
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RCU read-side critical sections, in case (2) the ->file_lock prevents
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any change from taking place, and finally, in case (3) the current task
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is the only task accessing the file_struct, again preventing any change
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from taking place. If the above statement was invoked only from updater
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code, it could instead be written as follows:
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file = rcu_dereference_protected(fdt->fd[fd],
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lockdep_is_held(&files->file_lock) ||
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atomic_read(&files->count) == 1);
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This would verify cases #2 and #3 above, and furthermore lockdep would
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complain if this was used in an RCU read-side critical section unless one
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of these two cases held. Because rcu_dereference_protected() omits all
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barriers and compiler constraints, it generates better code than do the
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other flavors of rcu_dereference(). On the other hand, it is illegal
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to use rcu_dereference_protected() if either the RCU-protected pointer
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or the RCU-protected data that it points to can change concurrently.
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There are currently only "universal" versions of the rcu_assign_pointer()
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and RCU list-/tree-traversal primitives, which do not (yet) check for
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being in an RCU read-side critical section. In the future, separate
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versions of these primitives might be created.
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